Don't forget the fake bullet holes. These easily add a few hp to the final figure, as the fake airflow through these fake holes reduces the drag of the vehicle.
Another touch is the false arm sticking out of the rear boot or hatchback, which is especially benefitial if you have a front engined car. There is a serious design flaw with placing the engine at the front, in that there isn't sufficient weight distribution, and dangling the false arm from the boot or hatchback helps to redress that balance. This is also why a lot of people drive with their seats as far back as possible, barely being able to touch the pedals with their feet, to help with weight distribution.